Aluminium base alloy



United States Patent 3,471,286 ALUMINIUM BASE ALLOY Frederick James Strong and Ian Frank Bainbridge, George Town, Tasmania, Australia, assignors to Comalco Aluminium (Bell Bay) Limited, Bell Bay, Tasmania, Australia, a corporation of Tasmania, Australia No Drawing. Filed May 27, 1966, Ser. No. 553,270 Claims priority, application Australia, June 1, 1965,

Int. Cl. C22c 21/02 U.S. Cl. 75-148 6 Claims ABSTRACT OF THE DISCLOSURE To modify and improve the casting properties of an aluminium-silicon-sodium alloy there is included in the composition a small but significant amount of an additive selected from the group consisting of lithium, titanium, bismuth, lead and the rare earth elements.

This invention relates generally to aluminium base alloys. More particularly it relates to an aluminium-silicon foundry alloy of improved casting characteristics.

The properties of the eutectic-type aluminium-silicon alloys modified by the addition of sodium are well known and these alloys are widely used for the production of castings. However, they possess certain disadvantages for this purpose in that: viscosity increases, and hence fluidity decreases, with increase of sodium content of the alloy; the alloy rapidly loses the modification properties due to loss of sodium from the melt by volatilization; and the alloy exhibits variable castability properties due to variable surface tension and fluidity properties. The alloy is therefore prone to the formation of shrinkage cavities within a casting and more specifically exhibits variable shrinkage forming properties from batch to batch of the allo The basic aluminium-silicon alloy modified in the normal manner with sodium has a surface tension that varies in the range 600 to 800 dynes per centimetre, rising rapid ly to higher values as the sodium is volatilized from the melt. Again the excessively thick and tenacious dross films formed on melts of this class of alloy, particularly where the amount of sodium exceeds 0.020%, are well known.

The object of this invention is to provide an aluminium base alloy that possesses improved castability properties by virtue of inherently low and substantially constant surface tension, by increased and substantially constant fluidity properties and by markedly reduced drossing characteristics.

According to the invention this object is achieved by the controlled addition of surface active substances which will modify and improve the surface tension, fluidity and drossing characteristics of alloys at present in use for casting. More specifically the invention provides an alloy which, in addition to aluminium and the usual impurities, includes 10% to by weight of silicon, a minor amount of sodium, and minor amounts of an additive or additives selected from the group consisting of lithium, titanium, bismuth, lead or any one or more of the rare earth elements and their chemical equivalents.

Still more specifically the invention provides a casting alloy which, in addition to aluminium and the usual impurities, comprises 10% to 15% of silicon, 0.003% to 0.040% of sodium and 0.003% to 0.040% of lithium to which may be added in individual amounts up to 0.50%, the total addition being not greater than 1%, titanium, load, bismuth, the rare earth elements and/or any alloy or chemical equivalent thereof.

Preferably the silicon content is between 11.5% and 13%, the sodium content between 0.005% and 0.020%, the lithium content between 0.005% and 0.015%, while of the impurities iron is preferably present in an amount not exceeding 0.40%, and copper, magnesium, manganese, zinc, nickel, chromium, tin calcium, vanadium, gallium and boron are each present in an amount preferably not exceeding 0.10%.

The invention also resides in a method or process for producing an aluminium casting alloy comprising the steps of adding to an aluminium-silicon alloy or to molten aluminium prior to the addition of silicon, a minor amount of sodium, and/or lithium or a chemical equivalent thereof, either in metallic form or combined as chemical compounds such as, for example, a sodium and/ or lithium salt, the sodium and lithium additive may be introduced into the molten aluminium silicon alloy either consecutively or simultaneously in a combined addition.

The method or process may also include the addition of one or more of the following elements either prior to or subsequent to the addition of the sodium and lithium or equivalent: titanium, bismuth, lead, the rare earth elements and any alloy or chemical equivalent thereof in individual amounts up to 0.50% the total not to exceed 1.0% and the preferred amount of each consistituent being 0.005%. Alloys of the above compositions exhibit a modified structure with much smaller additions of sodium than have hitherto been used for modification, and the modified structure is retained for a considerable time after the sodium has volatilized from the melt.

As surface tension has a profound influence on the modification effects, and hence on the mode of solidification which in turn governs the formation of shrinkage cavities, the alloys which fall within the scope of this invention have improved castability as compared with prior art alloys. By virtue of the lower sodium addition these alloys do not suflier from the marked increase in viscosity encountered with prior art alloys when sodium in amounts greater than 0.010% are added. This factor combined with low surface tension results in an improved and constant fluidity for the alloy. For example, this class of alloy normally exhibits a fluidity as measured by flow length due to gravity feed along a channel of 5 inches to 6 inches whereas the alloy of the invention yields a consistent flow length of 10 inches to 11 inches. This increased fluidity markedly assists in the filling of complex mould cavities, particularly those having thin sections, and assists in the feeding of the alloy during solidification thus reducing the formation of shrinkage cavities.

With respect to the formation of shrinkage cavities, a measure of this tendency in an alloy is provided by the centre line feeding resistance. The normal alloy in this class possesses a variable CFR of to depending upon sodium content and surface tension, whereas the alloy of the invention exhibits a constant CPR of 65% irrespective of sodium content. This low and constant CPR is a consequence of the low and constant surface tension, the improved fluidity and the retention of modification characteristics in the alloy. Again the lower sodium content and the high surface activity of the lithium and/or other additives markedly reduces the degree of drossing.

EXAMPLE One specific alloy according to the invention has the following composition:

the remainder being aluminium and the usual impurities.

This alloy has a tensile strength of 12.8 tons per square inch and exhibits an elongation of 10.5% before fracture. This alloy was submitted to laboratory tests designed to evaluate the castability properties of metals with the following results:

Fluidity, flow length ..-inches 10.8 Surface tension dynes/cm 545 Centre line feeding resistance percent-.. 64

After holding at 700 C. for one hour the microstructure was found to be fully modified.

Two further alloys, in accordance with invention and identified hereunder as alloy A and alloy B, were subjected to practical foundry testing under normal production conditions and compared with a conventional casting alloy identified hereunder as alloy C. The compositions of these alloys were as follows:

Percent Constituent Alloy A Alloy B Alloy Silicon 12. 30 12. 12. 50 Iron 0. 21 0. 28 0. 28 Sodium 0. 009 0. 004 0. 018 Lithium 0. 004 0. 005

The results of the tests showed that alloys A and B could be cast at a lower molten metal temperature (approximately 50 C. lower) than alloy C with equivalent mould fillings still being retained, and that the resultant increased production obtained with alloys A and B due to improved castability properties, approximated We claim:

1. A casting alloy consisting essentially of 10% to 15% silicon, 0.003% to 0.040% sodium and 0.003% to Percent Silicon 12.10-42.30 Iron 0.21-0.28 Sodium 0.009-0.004 Lithium 0.004-0.005 Aluminium Balance 5. A casting alloy as claimed in claim 1, which contains also at least one member of the group consisting of titanium, bismuth, lead and rare earth elements, the total addition not exceeding 1% by weight.

6. An alloy according to claim 5 wherein the amount of each additive does not exceed 0.50%

References Cited UNITED STATES PATENTS 1,387,900 8/1921 Pacz l48 1,410,461 3/1922 Edwards et al. 75l48 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75l43, 146, 147. 

